1. Field of the Invention
The present invention relates to a liquid crystal display device, and particularly to a grounding method of a shield conductive film formed on an opposed substrate in an IPS-type liquid crystal display device.
2. Description of the Related Art
In a liquid crystal display device, there are provided a TFT substrate on which pixel electrodes, thin-film transistors (TFTs) and the like are arranged in matrix, and an opposed substrate which faces the TFT substrate and on which color filters and the like are formed at positions corresponding to the pixel electrodes of the TFT substrate. In addition, liquid crystal is sandwiched between the TFT substrate and the opposed substrate. Further, the transmission of light of liquid crystal molecules is controlled for each pixel to form an image.
Since a liquid crystal display device is flat and lightweight, it has been widely used in various fields ranging from large-sized display devices such as TVs to cellular phones and DSCs (Digital Still Cameras). On the other hand, the characteristic of viewing angles is important in a liquid crystal display device. The characteristic of viewing angles is a phenomenon in which the brightness and chromaticity are changed when viewing the screen from a front or oblique direction. The characteristic of viewing angles is excellent in an IPS (In Plane Switching) type in which liquid crystal molecules are operated by an electric field in the horizontal direction.
In the IPS type, both of pixel electrodes and opposed electrodes are formed on the TFT substrate, and no electrodes are formed inside the opposed substrates. In such a structure, an electric field from outside enters a liquid crystal layer to become noise, deteriorating the image quality.
In order to prevent this problem, Japanese Patent Application Laid-Open No. Hei 9-105918 describes a configuration in which an external conductive film is formed outside an opposed substrate by sputtering ITO (Indium Tin Oxide) and the like and then is grounded, so that the inside of the liquid crystal display device is shielded. As grounding methods of the external conductive film, Japanese Patent Application Laid-Open No. Hei 9-105918 describes a method in which the external conductive film is connected to a metal frame through a conductive material and a method in which the external conductive film is connected to an earth terminal of a surrounding substrate through a cable. U.S. Pat. No. 6,034,757 exists as the corresponding patent of Japanese Patent Application Laid-Open No. Hei 9-105918.
In the method in which the external conductive film is directly connected to a metal frame through a conductive material, the external conductive film and a relatively-large connection area are required in consideration of accuracy of embedding the liquid crystal display device into a frame. However, a polarizing plate is arranged on the opposed substrate, and thus it is difficult to sufficiently provide an area where the external conductive film is connected to an outside earth terminal. In recent years, it has been required to increase a display area while keeping the outer shape of the liquid crystal display device at a predetermined value. In addition, a portion of the external conductive film that is not covered with the polarizing plate, namely, the exposed area of the external conductive film has been further decreased.
On the other hand, in the method in which the external conductive film of the opposed substrate is connected to an earth terminal of a wiring substrate or the like through a cable, it is difficult to secure the reliability of connection between the cable and the external conductive film of the opposed substrate.
An object of the present invention is to ground an external conductive film formed on an opposed substrate with a high degree of reliability in an IPS-type liquid crystal display device.